1. Technical Field
The present invention relates to a manufacturing method of a semiconductor device formed from a thin semiconductor wafer, such as a field stop (FS) insulated gate bipolar transistor (IGBT), and to a manufacturing apparatus thereof.
2. Related Art
Among manufacturing steps of a semiconductor device formed from a thin silicon wafer (hereafter simply called a wafer), such as an FS IGBT, after a surface structure of a semiconductor element is made on the upper surface of the wafer, there is a step of grinding the rear surface of the wafer, thus carrying out a reduction in film thickness (hereafter simply called “a reduction in thickness”). In the wafer rear surface grinding step, after a grinding protection tape, which is a surface protection tape which protects the upper surface of the wafer, is attached to the upper surface of the wafer (the upper surface of the surface structure), the rear surface of the wafer is ground to a desired thickness using a rear surface grinding apparatus. This grinding step is also called a backgrinding step. After the grinding is finished, the grinding protection tape is peeled from the upper surface of the wafer using a peeling tape.
In order to make it easier to peel the grinding protection tape from the upper surface of the wafer, a method of lowering the adhesion itself of the grinding protection tape is proposed. This is because it is necessary to prevent the wafer from being damaged when peeling the grinding protection tape from the wafer reduced in thickness.
Also, for example, in JP-A-2004-281430, it is disclosed that a wafer is held by a dicing tape (an adhesive tape), and a member supporting the lower surface of the dicing tape is provided so that the dicing tape will not go slack due to an ultraviolet irradiation for peeling, that the supporting member is configured of a material (glass or plastic) which transmits ultraviolet light, that the ultraviolet light is transmitted through the supporting member, and the dicing tape is irradiated with the ultraviolet light, and the like.
Also, in JP-A-6-224397, it is described that an ultraviolet (UV) irradiation curable tape (an ultraviolet peelable tape) is used as a surface protection tape, and it is possible to easily peel off the surface protection tape by irradiating the surface protection tape with ultraviolet light, and thereby weakening the adhesion thereof.
As another method, for example, in JA-P-4-225684, a semiconductor device manufacturing method is proposed whereby a light transmissive ultraviolet (UV) tape coated with an adhesive having the property of being reduced in adhesion when irradiated with ultraviolet light is attached to one surface of a wafer, and a thin portion is formed in the wafer.
Also, for example, in JP-A-63-30591, as a tape whose adhesion is reduced by ultraviolet light, an ultraviolet curable foam tape is also described, apart from the previously described ultraviolet irradiation curable tape.
In recent years, a use of an ultraviolet peelable tape in a wet etching, plate processing step, or the like, other than the rear surface grinding step, in order to protect an electrode surface formed on a wafer has been prevalent, and an ultraviolet peelable tape with higher adhesion has been developed. With this ultraviolet peelable tape, as the adhesion thereof is high, an ultraviolet irradiation amount necessary for peeling is ten times or more larger compared with that of an ultraviolet peelable tape used as the heretofore known grinding protection tape, and specifically, an ultraviolet irradiation amount of 1000 to 3000 mJ/cm2 is necessary for peeling.
FIGS. 14 to 22 are main portion manufacturing step sectional views showing a heretofore known semiconductor manufacturing method in the order of steps. Herein, a planar field stop (FS) IGBT shown in FIG. 24 is taken up as a semiconductor device. FIG. 24, being an enlarged view of a portion A of FIG. 22, is a main portion configuration diagram of a cell of the FS IGBT. In FIG. 24, reference numeral 51 is an n-type silicon substrate, 52 a p-well layer, 53 an n-emitter layer, 54 a gate insulating film, 55 a gate electrode, 56 an emitter electrode, 57 an interlayer insulating film, 58 an n-FS layer, 59 a p-collector layer, 60 a collector electrode, and 61 a surface structure.
Firstly, the surface structure 61 configured of the p-well layer 52, n-emitter layer 53, gate insulating film 54, gate electrode 55, emitter electrode 56, interlayer insulating film 57, and an unshown surface protection film (a polyimide film) covering a portion, other than the emitter electrode 56, electrically connected to the exterior is formed on the surface layer of an n-type wafer 1 (FIG. 14). The emitter electrode 56 at this stage is an aluminum electrode formed from an aluminum-silicon (AlSi) film 3.
Next, the surface structure 61 is attached to a grinding protection tape 2 (a backgrinding tape), and a rear surface 1a of the wafer 1 is ground, reducing the thickness of the wafer 1 (FIG. 15). The thickness of the wafer 1 is in the order of 80 μm for a product with a breakdown voltage of 600V, and in the order of 140 μm for a product with a breakdown voltage of 1200V.
Next, the grinding protection tape 2 is peeled off, an ion implantation of phosphorus, and an ion implantation of boron, into the ground rear surface 1a are carried out, and a thermal treatment is carried out, forming the n-field stop (FS) layer 58 and p-collector layer 59 (FIG. 16). The FS layer 58 is also called a buffer layer.
Next, an aluminum-silicon (AlSi) film 4, a titanium (Ti) film 5, a nickel (Ni) film 6, and a gold (Au) film 7 are deposited by sputtering on the p-collector layer 59, forming the collector electrode 60 which is a rear surface electrode (FIG. 17). At this stage, the rear surface side of the wafer 1 is curved in a concave form by the stress of the collector electrode 60. With a six inch wafer, a warp T thereof is up to as many as a dozen millimeters or so. The upper side of FIG. 17 is an enlarged view of a chip portion, and the lower side is a diagram showing the whole of the wafer 1 in such a way that the warp T of the wafer 1 can be seen.
Next, an ultraviolet peelable tape 8 is attached as a surface projection tape to the gold film 7 formed on the wafer rear surface 1a (FIG. 18). The warp is maintained even after the ultraviolet peelable tape 8 has been attached. As the ultraviolet peelable tape 8, there is an ultraviolet irradiation curable tape, an ultraviolet curable foam tape, or the like.
Next, an electroless nickel and substituent gold plating process is performed on the emitter electrode 56 (on the aluminum-silicon film 3) which is the surface structure 61 of the wafer 1, and a nickel film 9 and gold film 10 are formed deposited on the aluminum-silicon film 3 (FIG. 19). At this stage, the emitter electrode 56 is configured of the aluminum-silicon film 3, nickel film 9, and gold film 10. The rear surface 1a side of the wafer 1 is of a concave form after this step has finished, and in the case of a six inch wafer, the warp T is 2 mm to a dozen millimeters or so. Also, in the plating process, the collector electrode 60 formed on the rear surface 1a of the wafer 1 is protected by the ultraviolet peelable tape 8.
When an ultraviolet curable foam tape is used as the ultraviolet peelable tape 8, a foaming agent which generates a nitrogen gas in response to ultraviolet light 12 is contained in the ultraviolet peelable tape 8. In the electroless nickel plating process, as a strong alkaline solution with a pH of 12 or more and a strong acid with a pH of 1 or less are used as a pretreatment solution, a strong adhesion to the gold film 7 (the surface film of the collector electrode 60) is required of the ultraviolet peelable tape 8.
However, when the adhesion of the adhesive layer is increased, an ultraviolet irradiation amount of 1000 to 3000 mJ/cm2 is necessary for peeling after the plating process. This ultraviolet irradiation amount is ten times or more larger compared with the ultraviolet irradiation amount when using an ultraviolet peelable tape as the backgrinding tape (grinding protection tape 2), as heretofore described.
Next, with the wafer 1 placed on a support 11 with the surface structure 61 of the wafer 1 downward, and the ultraviolet peelable tape 8 in close contact with the wafer rear surface 1a upward, the ultraviolet peelable tape 8 in close contact with the warped wafer rear surface 1a is irradiated with the ultraviolet light 12 (FIG. 20).
Next, the ultraviolet peelable tape 8 in close contact with the rear surface 1a of the wafer 1 is peeled off (FIG. 21).
Subsequently, the wafer 1 is cut along dicing lines 17, forming chips 18 (FIG. 22).
As shown in FIG. 20, the wafer 1 to be irradiated with the ultraviolet light 12 is reduced in thickness by the backgrinding, and the rear surface 1a side (the upper side of the drawing) of the wafer 1 is warped in a concave form by the stress of the electrode 60 formed on the wafer 1. In the case of a six inch wafer, as the warp T is in the order of a dozen millimeters or so when it is large, the distance from the ultraviolet light source 13 to the wafer 1 changes in the plane of the wafer 1, and the ultraviolet illuminance varies in the plane of the wafer 1.
When a mercury lamp or a metal halide lamp is used as the ultraviolet light source 13, an ultraviolet illuminance of 30 to 100 mW/cm2 is easily obtained but, at the same time, the temperature of the wafer 1 rises to 100° C. or more in several seconds due to heat from the ultraviolet light source 13, and it may happen that an acrylic adhesive of the ultraviolet peelable tape 8 is altered, and a residue thereof remains on the peeled surface (rear surface 1a) of the wafer 1. In particular, the smaller the thickness of the wafer 1, the smaller the heat capacity of the wafer 1, meaning that the rise in temperature increases, a residue is more likely to remain, and a poor appearance occurs due to the residue.
FIG. 23 is a diagram showing the dependence of a wafer temperature on an irradiation time when using a metal halide lamp. A wafer 1, with a thickness of the order of 140 μm, placed on the support 11 is irradiated with the ultraviolet light 12 from above. A distance Lo between the metal halide lamp, which is the ultraviolet light source 13, and the wafer 1 is about 300 mm. The wavelength of the metal halide lamp at this time is 365 nm, and the ultraviolet illuminance is in the order of about 15 mW/cm2. The wafer temperature reaches 110° C., which exceeds the approximately 80° C. resistible temperature of the ultraviolet peelable tape 8, in an irradiation time of the order of 20 seconds. However, the ultraviolet irradiation amount at this time is 300 mJ/cm2, which is one third or less of the ultraviolet irradiation amount (1000 mJ/cm2) necessary for peeling.
Meanwhile, when a fluorescent tube type black light or ultraviolet light emitting diode with little heat generation is used as the ultraviolet light source 13, it is possible to suppress the rise in temperature of the wafer 1, but as there is no little heat generation from the lamp itself, it is not possible to bring the wafer 1 into contact with, or close to, the lamp. For this reason, it is necessary for the wafer 1 to be spaced some distance away from the lamp. For example, when the distance Lo from the wafer 1 is 20 to 50 mm, the ultraviolet illuminance is reduced to several mW/cm2. For this reason, there is a problem in that an irradiation time of several minutes to a dozen minutes or so is necessary in order to obtain an ultraviolet irradiation amount of 1000 mJ/cm2 or more necessary for peeling, and a throughput (a manufacturing man-hour) decreases.
This point will be more specifically described.
It is generally known that a relationship between illuminance and luminosity is given by the following equation.Illuminance=luminosity/(distance)2  Expression 1
It is understood from the above equation that, when the luminosity of the ultraviolet light source 13 is constant, the illuminance is reduced to one fourth when the distance from the ultraviolet light source 13 is increased twofold. The luminosity is luminous fluxes emitted from the ultraviolet light source 13 multiplied by area, and the illuminance is energy per unit time and unit area on a light receiving surface, which is represented by W/area. Also, an irradiation amount is energy per unit area on the light receiving surface, which is represented by J/area.
When a fluorescent tube type black light or an ultraviolet light emitting diode is used as the ultraviolet light source 13, as they have a low ultraviolet illuminance, it is necessary to reduce the distance Lo from the ultraviolet light source 13 to the wafer 1 in order to obtain the ultraviolet irradiation amount necessary for peeling.
A description will be given of a case in which, for example, a fluorescent tube type black light is used as the ultraviolet light source 13, the distance Lo is reduced to 20 mm in order to increase the ultraviolet illuminance, and the wafer 1 is warped 10 mm. In this case, the distance from the fluorescent tube type black light to a place (the peripheral end portion of the wafer 1) closest thereto is 10 mm, while the distance from the fluorescent tube type black light to a place (the bottom of the depressed portion) farthest therefrom is 20 mm. For this reason, the ultraviolet illuminance in the farthest place is one fourth of the ultraviolet illuminance in the closest place according to the previous equation. Meanwhile, as the distance can be reduced to 10 mm when the wafer 1 has no warp, when there is a warp, it is necessary that the irradiation time is made four times longer compared with when there is no warp. For this reason, when the wafer 1 has a warp, the throughput decreases, and a manufacturing cost increases.
Also, in JP-A-2004-281430 to JP-A-63-30591, it is not described that, when the wafer is originally warped, the warp is corrected, and the ultraviolet peelable tape is irradiated with the ultraviolet light, and peeled from the wafer.
As another method of correcting the warp of the wafer, a method is conceivable whereby the wafer is adsorbed by a porous chuck or an electrostatic chuck but, as it is also difficult in this case to adsorb the wafer without correcting the warp of the wafer at all, an auxiliary mechanism for correcting the warp of the wafer is necessary, meaning that there has been a problem in that a wafer support mechanism is complicated, increasing the cost.